Ink jet devices known in the art are divided in continuous stream devices and drop-on-demand devices.
In continuous stream devices, the ink is sprayed with a continuous stream of droplets through at least one nozzle. The droplets are electrostatically charged as they leave the nozzle and are deflected in flight by the application of high voltage pulses to a set of deflector plates. The trajectory of each drop, and then its contact position with the substrate to be printed, is accurately controlled.
In drop-on-demand devices, the ink is sprayed from a nozzle directly to a specific position of the substrate following the input of a digital system which positions the nozzle and provides the ejection of the droplets.
The present invention relates to an ink for drop-on-demand devices.
Drop-on-demand devices are mainly of three kinds. A first kind of device consists in a piezoelectric system which comprises a head fed with an ink which is ejected from a nozzle by a piezoelectric transducer which produces a pressure pulse. A second kind consists in an acoustic system employing the ability of a sound pulse of provoking the release of droplets from a liquid surface. A third kind consists in a thermal system which comprises a head equipped with a thermal resistance and fed with an ink. The thermal resistance causes the evaporation of the ink solvent (usually water) and the formation of a bubble, which in turn causes the ink ejection through the head nozzle.
In the field of printing on non-porous surfaces, like those of plastic, glass, metal, and composite materials, it is possible to use the above mentioned ink jet printing technologies.
The drop-on-demand ink jet printing technology shows some fundamental advantages in industrial printing, like a reduced ink consumption, a high image definition, and a low cost.
The fixing of the ink ejected by ink-jet technology on non-porous surfaces can be done with different methodologies. For example, the ink may be able to chemically attack the surface of the substrate so as to convey the colorant by means of a solvent within the substrate. Alternatively, the ink may comprise reactive components able to polymerise by thermal or photochemical treatments and to form a film which fixes and keeps the colorant on the surface of the substrate.
Both these techniques have drawbacks.
The first technique works only if the substrate to be printed is attacked by the ink solvent. Accordingly, the inks so formulated can be used only with plastic supports which are easily attacked by the ink solvents. In the case of printing on glass, metal or other surfaces with high chemical resistance it is necessary to resort to other techniques.
The second technique requires that the printed ink may be heated at high temperatures or exposed to UV-radiations to promote the polymerisation.
However, thermal polymerizable inks cannot be used with thermal ink-jet systems, as the temperature reached in the ejecting chamber would promote the polymerization of the ink components. Thermal polymerizable inks could be used with piezoelectric ink-jet systems, but still there are stability problems under storage which could result in the clogging of the printing head and require the storage at temperature below 0° C. Further, the use of high temperature for the polymerisation of the inks limits the type of substrates to be printed to those with high thermal stability.
The use of UV-polymerizable inks makes difficult the polymerisation of three-dimensional objects or surfaces which are not perfectly planar. Moreover, the total cost and the overall dimensions of a printer with a device for polymerisation is considerably higher. Besides that, reactive inks—where both reactive monomers or oligomers and polymerisation catalyst are mixed together in the same composition—are more likely to be unstable over time and to be more prone to clog the nozzles negatively affecting the printhead reliability.
Patents and patent applications disclosing reactive ink compositions having the above mentioned disadvantages are for example U.S. Pat. No. 5,380,769, U.S. Pat. No. 7,632,546, U.S. Pat. No. 7,563,489, US 2008/00384, and US 2008/0295731.
U.S. Pat. No. 7,632,546 discloses a radiation curable phase change ink preferably used in piezoelectric ink jet devices, comprising an ink vehicle that includes at least one gellant comprised of a curable polyamide-epoxy acrylate component and a polyamide component, and at least one colorant. U.S. Pat. No. 7,563,489 discloses radiation curable phase change ink preferably used in piezoelectric ink jet devices. The ink comprises an ink vehicle that includes at least one curable epoxy-polyamide gellant, and at least one colorant.
US 2008/00384 discloses a radiation curable phase change ink comprising an ink vehicle that includes at least one curable carrier, at least one gellant, at least one curable wax and at least one photoinitiator.
U.S. Pat. No. 5,380,769 discloses a reactive ink compositions comprising at least two reactive components, a base ink component and a curing component, that are applied to a receiving substrate separately. The base ink component includes an ink carrier, a compatible colorant, and a cross-linkable constituent, and the curing component is a cross-linking agent. Upon exposure of the base ink component to the curing component, at least a portion of the ink is cross-linked to provide a printed image that is durable and abrasion-resistant.
US 2008/0295731 discloses a reactive ink set including three mixtures of radically polymerizable monomers. A first mixture includes a peroxide, a second mixture includes a peroxide decomposition agent, and an optional third mixture that does not include a peroxide or a peroxide decomposition agent. The first mixture and the second mixture polymerize to form a solid ink on the substrate following jetting in the liquid state.